This invention relates to a method and apparatus for sealing articles. More particularly, the invention is used in package wrapping machines to seal heat sealable material such as thermoplastic overwrap to previously wrapped packages.
Articles such as food and tobacco products are commonly overwrapped with a transparent material to seal the packages as quickly as possible after the packages are manufactured. Typically, the overwrap material comprises a thermoplastic material which fuses upon application of heat. Various types of machines have been designed to apply and seal the overwrap, including machines wherein a heated plate is in position to be "wiped" with the part of the overwrap which is to be fused and sealed as the package is moved along a path adjacent the heated plate. Other packaging machines are intermittent thereby causing the package to pause in its movement while the heated plate is brought into contact therewith. An example of the latter type machine is in U.S. Pat. No. 3,979,881.
One of the desirable materials for use as the overwrapping material is "Hercules", marketed by Hercules, Inc. of Wilmington, Delaware. However, this material requires a fairly accurate regulation of the temperature of the heated plate, i.e. in the range of 125.degree.-165.degree. C. Accurate control of the temperature applied to the overwrap is not easily accomplished with those machines which "wipe" the overwrap with the heated plate, and uneven sealing and burning of the leading portion of the sealed area may result. In those machines exemplified by the aforementioned patents, accurate control of the temperature, package contact time and pressure is required to obtain a good seal and avoid burning or uneven sealing. The sealing function is accomplished by pressing a heated plate one time against the package. Under high speed operation, relatively higher temperatures must be used with corresponding shorter contact time. This may result in non-uniform operation and unsatisfactory seals along the seams of the overwrap.
Other typical prior art machines are exemplified in U.S. Pat. Nos. 3,984,963, 3,431,398, 3,692,611, 3,763,629 and 3,961,983.